There's Something in the Air
Corrosion Protection for Heat Exchangers Exposed to Air
Whether it's the underbody on cars, the hull on a ship, or steel pipes: corrosion is an ever-present phenomenon and has a major impact on the reliability and quality of metals and metal alloys. The corrosion behavior exhibited by the various groups of metals varies greatly. Air/water heat exchangers (such as those used in chillers and ventilation units) are usually made from aluminum or a combination of aluminum and copper. These metals have a natural oxide layer (passivation) and, as such, are considered to be relatively resistant to corrosion.
So, why should system operators even consider providing additional corrosion protection? Free cooling units and adiabatic evaporative cooling systems are playing an increasingly important role, in particular when it comes to optimizing the energy efficiency of precision climate control systems. As a result, refrigeration components such as microchannel coils are becoming mission-critical components which not only determine the system's energy efficiency, but also have a major impact on system availability.
Since all components are subject to corrosive processes of varying intensity depending on the environmental conditions in each case, it is often only a matter of time until measurable changes occur in the materials. By selecting the right type of corrosion protection together with a supplementary service strategy, operators can significantly extend the service life of their systems and avoid unscheduled downtimes.
Causes of Corrosion and Factors that Promote Corrosion
In general terms, the term 'corrosion' refers to the measurable reaction of a metal with its environment. Over time, this will affect the function of a component or system. Corrosive processes can be triggered by either chemical or electrochemical substances in the ambient air, or by contact between two different metals. Although corrosion essentially occurs everywhere, unfavorable environmental conditions can drastically speed up the natural degradation process.
In chilling systems, for example, pipes and fins in heat exchangers are continuously exposed to the outside air, which makes them fundamentally more susceptible to corrosion. In this respect, corrosion can have a severe impact on the performance of refrigeration systems. Heat exchangers may incur such severe damage that they become deformed or individual fins break. The resulting blockages in the air flow and the smaller heat exchanger surfaces then, in turn, lower the efficiency of the entire refrigeration system. In many cases, existing damage to the material structure is also irreversible and necessitates major repairs or even a replacement.
One of the most common causes of corrosion in cooling technology is general oxygen corrosion and acid corrosion, both of which are called surface corrosion. General oxygen corrosion is caused by a metal reacting with oxygen and forming oxides. With copper and in particular with aluminum, the natural oxide layer protects the inside of the metal to a certain extent, meaning natural oxidation can even be considered beneficial in these cases.
However, acid corrosion is much more aggressive. In locations where major industrial processes take place, emissions can significantly speed up the corrosion rate. Compounds such as nitrogen oxides and sulfur oxides, as well as ammonia, chlorides, and carbon monoxide, react with copper and aluminum and form microscopic cavities in the material. This process is called "pitting" and can cause severe corrosion damage in only a few months.
The greatest risk factors in terms of speeding up the corrosive degradation process can be found in locations with heavy industry or heavy traffic, or in the vicinity of agricultural land. Surface corrosion can cause severe damage in all types of heat exchanger – with the risk of the system working less efficiently or, in extreme cases, even failing completely.
Furthermore, as soon as more than one type of metal is used in a refrigeration component, there is always the risk of galvanic corrosion in a system. In conjunction with an electrolyte such as salt water, ions from a "less noble" metal start to flow in the direction of a "more noble" metal. At some point, this will damage the metal that is releasing ions. This phenomenon particularly affects copper/aluminum combinations as well as combinations of metals in environments subject to a lot of salt water. Aluminum microchannel coils, combined with copper pipes, are particularly susceptible to major damage caused by galvanic corrosion. Any fractures or blockages in the fine channels then cause partial pressure drops or even cause refrigerant to leak out.
The risk of corrosion is also increased by other factors such as the use of sub-standard metals. In this case, fractures and cavities in the surface may capture water and accelerate the degradation process. A warm, humid climate and high fluctuations in temperature will also make it easier for corrosion to occur.
Methods of Corrosion Protection and Their Benefits
Thanks to state-of-the-art process engineering, we nowadays have extensive possibilities for effectively protecting heat exchangers against corrosion. Each method has its own advantages and disadvantages. For example, the use of thicker aluminum fins will provide protection against premature fin fractures and is even relatively inexpensive to implement. The additional material slows down the impact of corrosive processes on the material structure, thereby prolonging the time until a fracture occurs.
However, when the system is in operation the wider fins reduce the air flow and the overall efficiency of the heat exchanger. A better option would be the use of Cu/Cu coil fins. They enable the impact of galvanic corrosion to be completely eliminated as the pipes and fins in the heat exchanger are made entirely of copper. What is more, the heat exchanger also retains its heat conductivity properties. Its main disadvantages are that higher material costs are incurred, and that it offers insufficient protection against oxygen corrosion.
In addition to structural measures, coating techniques can also help to protect against corrosion. In this context, state-of-the-art top coats are applied directly to the surface of the heat exchanger components using pressurized spraying systems. A non-reactive resin such as epoxy is often used here to protect the metal components against acidic solutions in the atmosphere. The sprayed-on coating provides protection against all types of corrosion and is relatively inexpensive compared against other types of coating. However, the relatively thick layer produced as a result is less energy-efficient and reduces the efficiency of the cooling system as it impairs the system's heat conductivity.
Additional pressure drops also increase the fans' power consumption. That is why special epoxy resins such as Blygold use aluminum-pigmented polyurethane. When used accordingly, the substance itself offers an outstanding level of protection. It can also be applied very evenly, meaning the individual layers are very uniform.
However, a known problem that occurs with all sprayed-on coatings is the risk of gaps in the top coat. Due to the spraying technique, and in particular on thick heat exchangers, these gaps can never be ruled out entirely and will make it easier for spot-type corrosion to occur. As a result, sprayed-on coatings are recommended for locations with conventional installation conditions. However, due to the weaknesses in the method as described above, they are less suited to locations with very high environmental loads.
In contrast to sprayed-on coatings, cathodic dip painting stands out by ensuring very uniform layers. With this method, the heat exchanger is electrically charged and then fully immersed in a chemical bath. As a result, the coating adheres evenly to all surfaces. If the components are then mounted with due care and attention, no gaps in the protective coating should be expected. In addition to ensuring a very uniform result, i.e. layers of very even thickness, the cataphoresis also produces the thinnest protective layer of all the coating methods and has only a minor impact on the heat conductivity.
The special ElectroFin method additionally uses cationic epoxy polymer, which is applied using the cathodic dip painting process and provides outstanding protection against all types of corrosion. However, the drawback of this technically complex coating process is that it is much more expensive.
Overview of the Various Corrosion Protection Methods
|Method||Uniformity||Layer thickness||Salt spray test||Loss of heat conductivity||Additional costs|
|Cu/Cu coil fins||-||-||-||~0%||Medium|
|Sprayed-on coating (epoxy)||OK||50-70 μm||1,500+ hours||~3-5%||Medium|
|Sprayed-on coating (Blygold)||Good||25-30 μm||11,000+ hours||0-3%||Very high|
|Cathodic dip painting (ElectroFin)||Very good||15-25 μm||6,000+ hours||0-1%||High|
The comparison of the various corrosion protection methods above enables a number of key conclusions to be drawn for use in practice. A sprayed-on coating with aluminum-pigmented polyurethane (Blygold) offers the most effective protection on account of the very good results obtained in the salt spray test. However, it still entails the risk of spot-type gaps occurring in the corrosion protection system.
Therefore, despite obtaining a worse result in the salt spray test, cathodic dip painting (e-coating) can be considered an impressive solution – in particular because of the more reliable coating technique. Although a coating with epoxy resin performs worse than other methods when it comes to a reduction in performance and its ability to protect against corrosion, it is a much less expensive method.
Structural measures such as Cu fins/Cu pipes and wider aluminum fins are relatively simple to implement in manufacturing terms. Although wider aluminum fins are the most affordable solution, they are also the least reliable. For this reason, thicker aluminum fins should preferably be used in combination with other types of protection. Cu/Cu coil fins are relatively expensive and offer only a limited benefit compared against aluminum fins. As they primarily protect against galvanic corrosion, they are especially suitable for systems located in close proximity to the sea.
Location Conditions and Protective Methods
|Location conditions||Corrosion factors||Suitable protection method|
|Airport Heavy traffic||General corrosion fom NOx, SOx and CO||Sprayed-on coating or e-coating|
|Industry||General corrosion from NOx, SOx, ammonia, chlorine etc.||Sprayed-on coating or e-coating|
|Close proximity to the sea||Galvanic corrosion caused by salt water in a high concentration||Cu/Cu coil fins|
|Coastal area||Galvanic corrosion caused by salt water in a low contentration||Cu/Cu coil fins, sprayed-on coating or e-coating|
|Agriculture||General corrosion from NOx, SOx and ammonia||Sprayed-on coating or e-coating|
|Combination of location conditions||Galvanic and general corrosion||High-quality sprayed-on coating or e-coating|
Integrating Corrosion Protection into Maintenance Planning
Nowadays, chilling systems and air-conditioners carry out key, mission-critical tasks over a long usage period of 10 to 15 years. In the IT and telecommunications industry, effective air conditioning is one of the key factors for ensuring high availability of equipment and high energy efficiency. An appropriate service strategy also plays a crucial role in maintaining the system's performance. For this reason, a comprehensive maintenance strategy should include not only the mandatory leak tests but also corrosion protection methods. Inspecting the system regularly will not only maintain the efficiency of the air/water heat exchanger, it will also significantly prolong the service life of the entire system.
In order to improve the corrosion protection for the long term, it is recommended to clear coarse dirt such as leaves, pollen, and dust from the heat exchanger surfaces at regular intervals using an industrial vacuum cleaner. The coating must then be thoroughly inspected for contamination and damage. Fine dirt and accumulations are best removed using a high-pressure cleaner or water from a hose. If you use additional detergents, also ensure that the detergent and coating are compatible with one another. The respective manufacturers will provide precise instructions in this respect.
However, in many cases simply using warm or hot water without any chemical additives will also suffice. As a basic rule, always clean the unit in the opposite direction to the air flow. Moreover, with many coating materials and techniques the coating needs to be re-applied after a few years. This is often done using spraying technology, and can be performed directly at the installation site. The respective manufacturer will likewise provide the recommended intervals for doing this.